This invention relates to the casting of steel strip in a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
When casting steel strip in a twin roll caster the casting pool will generally be at a temperature in excess of 1550° C. It is necessary to achieve very rapid cooling of the molten steel over the casting surfaces of the rolls in order to obtain solidification and form solidified shells in the short period of exposure on the casting surfaces to the molten steel casting pool during each revolution of the casting rolls. Moreover, it is important to achieve even solidification so as to avoid distortion of the solidifying shells which come together at the nip to form the steel strip. Distortion of the shells can lead to surface defects known as “crocodile skin” surface roughness. Crocodile skin surface roughness is illustrated in FIG. 1, and involves periodic rises and falls in the strip surface of 40 to 80 microns, in periods of 5 to 10 millimeters, measured by profilometer. Even if pronounced surface distortions and defects are avoided, minor irregularities in shell growth and shell distortions will still lead to liquid entrapment in discrete pockets, or voids, between the two shells in the middle portion of the steel strip. These voids are generated as the entrapped liquid solidifies, and cause a porosity in the steel strip observed by x-ray as shown in FIG. 2 herein and in FIG. 2b of our paper entitled “Recent Developments in Project M the Joint Development of Low Carbon Steel Strip Casting” by BHP and IHI, presented at the METEC Congress 99, Düsseldorf Germany (Jun. 13-15, 1999). This necessitates in-line hot rolling of the strip to eliminate the porosity since the strip cannot otherwise be used even as feed for cold rolling because of cracks generated by the voids and potential breakage of the strip under tension.
It has hitherto been thought that such internal porosity was inevitable in as-cast thin cast strip, which needed to be eliminated by in-line hot rolling. However, after carefully considering the factors which may lead to uneven solidification and extensive experience in casting steel strip in a twin roll caster with control over those various factors, we have determined that it is possible to achieve more even shell growth to avoid crocodile skin surface roughness, and also, avoid significant liquid entrapment and thus substantially reduce porosity to virtually zero.
According to the present invention, there is provided a method of producing thin cast strip with low surface roughness and low porosity comprising the steps of:
assembling a pair of cooled casting rolls having a nip between them and with confining closure adjacent the ends of nip;
introducing molten steel having a total oxygen content of at least 100 ppm and preferably below 250 ppm between the pair of casting rolls to form a casting pool between the casting rolls at a temperature such that the majority of oxide inclusions formed therein are in liquidus state;
counter-rotating the casting rolls and transferring heat from the molten steel to form solidified shells on the surfaces of the casting rolls such that the shells grow to include oxide inclusions relating to the total oxygen content of the molten steel and form steel strip free of crocodile surface roughness; and
forming solidified thin steel strip through the nip between the casting rolls from said solidified shells.
Although also useful in making stainless steel, the method has been found particularly useful in making low carbon steel. In any case, the steel shells may have manganese oxide, silicon oxide and aluminum oxide inclusions so as to produce steel strip having a per unit area density of at least 120 oxide inclusions per square millimeter to a depth of 2 microns from the strip surface. The melting point of the inclusions may be below 1600° C. and preferably is about 1580° C. Oxide inclusions comprised of MnO, SiO2 and Al2O3 may be distributed through the molten steel in the casting pool with an inclusion density of between 2 and 4 grams per cubic centimeter.
Without being limited by theory, avoidance of crocodile skin surface roughness and lower porosity is believed to be provided by controlling the rate of growth and the distribution of growth of the solidifying metal shells during casting. The primary factors in avoiding shell distortion have been found to be caused by a good distribution of solidification nucleation sites in the molten steel over the casting surfaces, and a controlled rate of shell growth particularly in the initial stages of solidification immediately following nucleation. Further, we have found that it is important that before the solidifying shells pass through the ferrite to austenite transformation, the shells reach sufficient thickness of greater than 0.30 millimeters to resist the stresses that are generated by the volumetric change that accompanies this transformation, and further that transformation from ferrite to austenite phase occur before the shells pass through the nip. This will generally be sufficient to resist the stresses that are generated by the volumetric change that accompanies the transformation. Typically, with the heat flux on the order of 14.5 megawatts per square meter, the thickness of each shell may be about 0.32 millimeters at the start of the ferrite to austenite transformation, about 0.44 millimeters at the end of that transformation and about 0.78 millimeters at the nip.
We have also determined that crocodile skin roughness is avoided by having a nucleation per unit area density of at least 120 per square millimeter. We believe such crocodile skin roughness is also avoided by generating controlled heat flux of less than 25 megawatts per square meter during the initial 20 millisecond solidification in the upper or meniscus region of the casting pool to establish coherent solidified shells, and to ensure a controlled rate of the growth of those shells in a way which avoids shell distortion which might lead to liquid entrapment in the strip.
A good distribution of nucleation sites for initial solidification can be accomplished by employing casting surfaces with a texture formed by a random pattern of discrete projections. Said discrete projections of the casting surfaces may have an average height of at least 20 microns and they may have an average surface distribution of between 5 and 200 peaks per mm2. In any event, the casting surface of each roll may be defined by a grit blasted substrate covered by a protective coating. More particularly, the protective coating may be an electroplated metal coating. Even more specifically, the substrate may be copper and the plated coating may be of chromium.
The molten steel in the casting pool may be a low carbon steel having carbon content in the range of 0.001% to 0.1% by weight, manganese content in the range of 0.01% to 2.0% by weight and silicon content in the range of 0.01% to 10% by weight. The molten steel may have aluminum content of the order of 0.01% or less by weight. The molten steel may have manganese, silicon and aluminum oxides producing in the steel strip MnO.SiO2.Al2O3 inclusions in which the ratio of MnO/SiO2 is in the range of 1.2 to 1.6 and the Al2O3 content of the inclusions is less than 40%. The inclusion may contain at least 3% Al2O3 
Part of the present invention is the production of a novel steel strip having improved surface roughness and porosity by following the method steps as described above. This composition of steel strip cannot, to our knowledge, be described other than by the process steps used in forming the steel strip as described above.
In order that the invention may be more fully explained, the results of extensive experience in casting low carbon steel strip in a twin roll caster will be described with reference to the accompanying drawings in which: